Technological Breakthrough in MSSL Milli-Kelvin Cryocooler

14 June 2012

Figure 1: Chromium Potassium Alum refrigerant in the cryostat for testing

A 22 gram Chromium Potassium Alum (CPA) paramagnetic refrigerant has been demonstrated to cool from 4 Kelvin to 178 millikelvin in only 30 seconds; an unprecedented achievement made possible by detailed thermal modelling and the development of a superconducting magnet capable of ramping from zero to two Tesla (20,000 gauss) in 30 seconds and vice versa. This is a key technological breakthrough in MSSL’s EPSRC funded millikelvin cryocooler development.

The CPA refrigerant (pill) is the first part of MSSL’s miniature millikelvin cryocooler to be tested and is one 20th of the size of previous CPA pills used at MSSL and has been demagnetised 50 times faster. This is a huge leap in terms of the CPA pill design and magnet technology (magnets provided by Scientific Magnetics, Abingdon).

The miniature millikelvin cryocooler project was started in 2010 with the goal to design and build the first millikelvin cryocooler module ; a tandem continuous ADR (operating from a 4 K interface) comprising of two double ADRs (dADRs) capable of providing continuous cooling at any user specified temperature between 100 mK and 4 K. It uses ten 2 Tesla superconducting magnets (which can ramp in 30 seconds) and will be the first ADR cooler to use only single crystal tungsten magnetoresistive heat switches allowing the cryocooler to be very fast cycling and compact (355 mm high, 120 mm wide, 56 mm deep with a mass of 8.3 kg). Without the successful demonstration of the CPA pill performance, the design of the cryocooler would have been significantly affected.

A CPA pill consists of CPA crystals and a gold plated copper thermal bus which provides the thermal link between the crystals and the cold stage, on which the experiment is mounted. The thermal link between the crystals and the thermal bus is of paramount importance; an excellent thermal link is required to ensure that there is no thermal lag between the cold stage and the crystals. This is achieved by having a large surface area of gold plated copper wire in direct contact with the CPA crystals (the crystals are in fact grown on the wires) but therein lies a problem: copper, which is needed because of its high thermal conductivity is subject to eddy current heating; eddy currents are induced in a conductor when it is exposed to a changing magnetic field and the magnitude of these currents is proportional to the rate of change of the magnetic field i.e. the greater the rate of change of magnetic field, the larger the eddy currents. This causes a problem in cryogenic systems as eddy currents generate heat. In other ADRs, the CPA pills are demagnetised over 10s of minutes therefore eddy current heating is not an issue. For the millikelvin cryocooler however, eddy current heating becomes a major factor in the design due to the small size of the CPA pill (the smaller the pill the smaller the heat loads it can deal with) and the 30 second magnet ramp times. The design of this CPA pill is therefore very different to previous pills and was devised after extensive magnetic and thermal modelling.

Figure 2: Temperature of the CPA pill during two magnetisation and demagnetisation cycles.

Testing of the CPA pill is the first stage in the testing of the millikelvin cryocooler. The results are shown in Figure 2. Two cycles are shown, each consisting of a magnetisation phase, cooling phase (magnetisation generates heat within the pill which has to be extracted to the 4 K bath) and demagnetisation phase. Each cycle takes less than 2.5 minutes whereas typically in other systems this would take at least several hours. The figure also shows the excellent thermal link between the crystals and the thermal bus giving confidence in the full cryocooler design and validating the advanced sub-second dynamic thermal modelling conducted at MSSL in the millikelvin to 4 K temperature region.

While this project is aimed at ground based instrumentation, it is well suited for adaptation to space thereby easily providing continuous cooling for sub-millimetre and X-ray cryogenic detectors.

The millikelvin cryocooler is due to be completed in October 2012.

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